When you start the Terminal application, you are looking at a program called a shell. The shell is a program that looks like the traditional model of a window into which you type commands and see their output. The big transition from Mac OS 9 to OS X was the use of a version of the Unix operating system for its internal operation. The shell has been the traditional text-based mode of interaction with Unix from its beginning.
Different versions of shell programs have been created since programmers started running Unix in the 1970s. The shell that runs by default in Mac OS X is called the Bash shell. All shell versions have a core set of commands. Some of these are described in the next tutorial. We will be using the Terminal throughout the class, and we'll do many of the exercises together.